Note 3 - Net Income Per Share - Basic and Diluted Net Income Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
Net income (numerator):
Net income	$ 12,582	$ 9,276
Shares (denominator):
Weighted average number of common shares outstanding used in basic computation (in shares)	20,408	20,293
Common shares issuable upon vesting of restricted stock (in shares)	31	145
Common shares issuable upon conversion of preferred shares (in shares)	10	10
Shares used in diluted computation (in shares)	20,449	20,448
Net income per common share
Basic (in dollars per share)	$ 0.62	$ 0.46
Diluted (in dollars per share)	$ 0.62	$ 0.45